In many instances, agricultural grain products must be stored for an extended period of time prior to being used. However, prior to storage, it is necessary to dry the grain to a condition in which it is less subject to molding or other deterioration. Accordingly, it has become known to remove moisture from grain by passing the grain through a grain dryer prior to storage.
Tower grain dryers are well known. Generally, they comprise a vertical tower having a cylindrical shape with an annular grain plenum. The outer wall of the plenum is generally cylindrical. An inner cylindrical wall coaxial with the outer wall is spaced inwardly from the outer wall. Generally, the inner cylindrical wall has a diameter of about two feet less than the outer wall, resulting in an annular plenum between the walls having a thickness of about 12 inches. The plenum provides a vertical grain path between the two walls. The walls are constructed from perforated stainless steel screens to be porous such that heated air from within the plenum may be forced through the walls and through the grain in the plenum.
A typical annular plenum is divided into a series of vertical columns by dividers which are circumferentially spaced one to two feet apart in the plenum. Grain loaded in the top of the tower descends under force of gravity thought the vertical columns in the plenum.
As heated air moves through the grain, moisture is removed. Dried grain is continuously discharged from the lower end of the plenum. Additional grain to be dried is loaded into the upper end of the drying path.
To control the amount of moisture removed from the grain, it is necessary to precisely control the flow rate of the grain through the grain columns of the plenum. Grain in the grain columns exposed to heated air for an extended period of time may become too dry. Grain that passes quickly through the grain columns may retain an undesirable amount of moisture.
Heated air is supplied to the grain by means of one or more burner/blower assemblies. Heated air is forced from within the radially inner wall of the plenum through the plenum's porous or perforated inner wall, through the grain in the drying paths, and finally through the porous outer plenum wall, carrying away moisture from the grain.
Expensive and elaborate sweep systems have been developed to remove grain from drying towers. Sweep systems are located at the bottom of the plenum and remove grain from the bottom of the grain columns when dried to a desired moisture content. As is well known, sensors in the grain tower determine the moisture content of the grain and signal the grain removal system, typically a sweep system, to remove certain amounts of grain from the bottom of the vertical columns. However, for various reasons, grain descending through the various columns does not dry at the same rate, with some grain being over dried and some grain being under dried.
Currently available grain dryers can be inefficient for several reasons. The primary source of inefficiency is imprecise drying due to uneven drying among the various vertical grain paths. This can result from various factors, such as wind and sun affecting some vertical grain columns more than others. Sweep systems at the bottom of a typical grain dryer sweep grain generally evenly from the adjacent columns even though grain in adjacent columns may have different moisture content.
Another possible source of inefficiency is that the perforated plenum walls or screens often clog from the grain or grain shells, etc. To maintain dryer efficiency, it is necessary to clean the screens periodically. Typically this is done manually. Large grain towers typically have catwalks vertically spaced approximately every 10 feet for this purpose. Catwalks add capital cost to the dryers, and manual cleaning adds operating or maintenance costs. Frequent cleaning of the plenum is necessary for maximum efficiency. Unfortunately, because of the high cost of cleaning, grain towers are not always operated at maximum efficiency.
What is needed is a grain tower which more efficiently dries grain and which can be less expensively manufactured and maintained.